Ferguson confirmed he struck a gentlemen’s agreement with Ronaldo in the summer of 2008 giving him permission to leave Manchester United if he stayed one more season.

That was despite the Scot being deeply unhappy with the behaviour of then-Real president Ramón Caldéron, whose controversial courtship of Ronaldo eventually paid off when he signed the forward for £80 million.

Writing in his autobiography, Ferguson hailed Ronaldo as the most naturally gifted player he managed, recounting how the Portuguese developed from a raw talent with a habit of showboating into the most expensive footballer in history.

He revealed how his assistant, Carlos Queiroz, had told him about Ronaldo upon first joining the club in 2002. Ferguson said that when he scouted a 17-year-old in Lisbon, he felt “the biggest surge of excitement” of his managerial career.

Ferguson also detailed how he managed the potential fallout between Ronaldo and Wayne Rooney following the 2006 World Cup. Rooney had been sent off in England’s quarter-final defeat by Portugal, with Ronaldo encouraging the referee to issue a red card to his club-mate.

Ronaldo, says Ferguson, regretted the incident but feared it might be impossible to return to United. Rooney saved the day by calling Ronaldo several times to reassure him and even suggested the two of them got together for a joint interview to show that there were no hard feelings.

The England-Portugal game also convinced Ferguson to sign Owen Hargreaves, a decision he branded a “disaster” for United. Ferguson accused the injury-plagued midfielder of lacking the determination to overcome his fitness problems and revealed the club considered suing him over his claims that he had been let down by their medical staff.

The Scot dubbed Hargreaves one of his most disappointing signings, having watched him make just 27 appearances in four years after earmarking him as Roy Keane’s successor. Most of those years were spent battling knee and calf problems and seeking cures in the likes of Germany, America and Canada.

Ferguson said he had doubts about Hargreaves even before the player completed his £17 million move from Bayern Munich and claimed there was something about him he did not like. The Scot accused Hargreaves of choosing the easy option in training, although he insisted he had no complaints about his performances when the midfielder actually did play.

Ferguson described as “rubbish” an allegation by Hargreaves that he asked not to be selected for the infamous game against Wolverhampton Wanderers in 2010 when he lasted just five minutes due to a hamstring injury.

The following year, Hargreaves accused United of treating him like a guinea pig for his tendonitis and various knee problems. Ferguson revealed the club took legal advice over the matter but said the doctor involved was not sufficiently offended to pursue the case.